Any tailshaft for Datsun 1200 to suit an L-series long 5-speed will be a custom one-off tailshaft.

Except for very old tailshafts (1971 or earlier) Datsun shafts have non-servicable u-joints.

Datsun shafts also narrow down at the ends, hence they can't be shortened in the industrial manner. Although machine shops can do it, driveline shops touch them.

Which is why most folks have a new shaft built.